This invention relates to heat exchanger assemblies and more particularly to an improved fin array design for use in a variety of heat exchanger assemblies and a method of making the fin array.
FIG. 1 illustrates a prior art heat exchanger assembly in the form of a condenser typically used in air conditioning units for vehicles. The heat exchanger assembly 10 includes a pair of opposed, spaced, generally parallel headers 11 and 12. The headers 11 and 12 each define a series of generally parallel slots or openings 13 for receiving the ends 14a and 14b of tubes 14 that extend in fluid communication between the headers 11 and 12. Each of the headers 11 and 12 includes a fitting 15 and a cap 16. The fittings 15 operate as either an inlet or outlet for circulation of fluid through the headers 11 and 12 and tubes 14. The fittings 15 can be operatively connected, such as by tube 17 or other appropriate tubing, to a heat exchanger system such as for a air conditioning unit for a vehicle. The heat exchanger assembly 10 also includes channels or flanges 18 and 19 in order to provide rigidity to the structure.
A plurality of elongated serpentine fins 20 extend between the headers 11 and 12 along each of the heat exchanger tubes 14. Each of the fins 20 follows a serpentine pattern and has rounded crests that are alternately connected to the top and bottom tubes 14 by a process such as brazing.
It is well known in the art that the efficiency of a heat exchanger assembly is mainly limited by the heat flux between the fins and the ambient air, which receives the heat from the system or transmits heat into the system depending upon the application. For example, in the case of mechanical refrigeration systems, it is known that the heat flux per unit of area between the tube walls and refrigerant or between the tube walls and fins is very high relative to the heat flux per unit area between the surrounding air and the fin and tube surfaces. It is also known in the art that the portion of the fin that first cuts through the air has the highest heat flux per unit area.
To improve heat flux between the fins and the ambient air, many heat transfer systems employ a fan to move more air per unit of time across the fins. As another example, moving vehicles such as automobiles typically position the air conditioning condenser on the front of the car to provide maximum air flow across the fin and tube surfaces.
In another system to improve heat flux between the fins and ambient air, the fins are manufactured to include small louvers in each fin that catch the air and force the air to flow past or over the heated or cooled fin surfaces. A fin array 21 including louvers on the fins is shown in the prior art fin assembly of FIG. 2. The fin array 21 is folded in a serpentine pattern to form a series of alternating upper and lower crests 22 and a plurality of individual fins 23. Each of the individual fins 23 includes a plurality of louvers 24.
The elongated fin array 21 is typically manufactured from strips of metal, such as copper or aluminum, that are run through rotary cutting dies that shape the openings in a strip, shape the louvers by pushing them inward or outward from the strip, and then fold the fins using a xe2x80x9cstar wheelxe2x80x9d style roller which imparts a rounded bend to the fin stock. The fin array 21 including louvers 24 on the fins 23 improves the heat flux as compared to traditional non-louvered fins. However, the louvered fins are less than optimal for maximizing heat flux between the fins and ambient air and are difficult and expensive to manufacture.
For example, the louvers 24 on the fins 23 do not extend across the entire length of the individual fins due to the rounded bend area at crests 22 and thus form bypass passageways labeled 25 in FIG. 2. Air can thus pass entirely through the fins 22 at bypass portions 25 without encountering the louvers 24 or substantially contacting the fins 23.
In the louvered fin array 21, the louvers 24 are also aligned directly behind each other such that the air tends substantially to contact only the first row or two of the louvers 24. Thus, the louvers 24 toward the back of the fin set do not xe2x80x9cseexe2x80x9d fresh air since they are in the shadow of the first louvers.
The louvered fin array shown in FIG. 2 is typically manufactured by cutting the fins in a traditional shearing die technique. With most metals such as copper or aluminum, those skilled in the art know that large amounts of lubrication are required for shear cutting of the material in order to prevent heat build-up in the cutting tools. However, the lubricating oils must be substantially removed from the fins after the cutting process so that the fins are clean for brazing the fins to the tubes. The process of removing the lubricating oils from the fins is an expensive process and may result in environmentally dangerous byproducts.
This manufacturing process also commonly results in relatively large fin height variations that can lead to poor bonding between the fins and tubes. As a consequence of tolerance build up, added to run by run in the full assembly process, the rounded upper and lower crests of the fin array may not allow for complete fin to tube contact if the tubes are thinner than normal or if the fins have been folded with too small of a height. Poor bonding between the fins and tubes can dramatically decrease the efficiency of the entire heat exchanger assembly. If, on the other hand, fins have been folded with too great a height and/or tubes are thicker than normal, then some runs of the fins may be crushed out of shape allowing increased (or decreased) by-pass (or breakage). Both of which are detrimental to heat transfer.
An important aspect of this invention lies in providing an improved fin array for a heat transfer assembly that provides improved heat flux between the fins and the ambient air and that permits more efficient and economical manufacturing than prior art fin arrays including louvers. The fin array of the present invention comprises an elongated serpentine one-piece fin member having top and bottom base portions connected together by fin sets extending between adjacent ones of the top and bottom base portions. The fin sets each include a plurality of individual fins having side edges facing generally perpendicular to a longitudinal length of the one piece fin member. The side edges of the fins are also longitudinally offset with respect to each other to improve heat flux with the passing air.
The fin sets are divided into a plurality of individual fins that have offset sides edges which greatly increase the heat flux of the entire fin member. The side edges of the fins typically provide the greatest amount of heat flux and the offset nature of the side edges of the fins maximizes this heat flux since each of the edges sees fresh air.
The top and bottom base portions of the fin unit extend respectively in top and bottom planes and are generally flat. The flat nature of the top and bottom base portions permits solid bonding and increased surface area in contact with the heat exchanger tube to increase overall heat transfer. The flat configuration of the top and bottom base portions or crests also provides a better and more stable connection than prior art fins having rounded crests.
The top and bottom base portions generally comprise elongated, flat sections that extend transversely at an angle with respect to the longitudinal length of the fin member. The base portions are formed of staggered sections, that may comprise either rectangles or squares, in order to longitudinally offset the side edges of the fins. This permits dense packaging of the fins and their side edges to fully meet and engage oncoming air in order to improve heat flux.
In that regard, the fins preferably extend at an angle of 90xc2x0 with respect to the top and bottom base portions. The fins then extend completely between the top and bottom heat exchanger tubes to maximize heat transfer. This configuration also prevents formation of xe2x80x9cpassage waysxe2x80x9d that could otherwise allow air to pass through the fin without contacting any of the fin or tube surfaces.
The inventive fin array of the present invention can advantageously be manufactured without the use of shearing devices or associated lubrication oils, which otherwise can make the manufacturing process unduly complicated, expensive, and harmful to the environment. In particular, the method of manufacturing the inventive heat array includes providing a flat sheet of fin stock and then positioning the fin stock between chisel and an anvil. The chisel is then press or impacted into the fin stock so that the chisel penetrates the fin stock to between about 40 to 90 percent of the thickness of the fin stock in order to define a cut pattern along the length of the fin stock. In most metallic fin stock materials, the chisel shape need not penetrate entirely through the material since lateral forces applied by the chisel to the fin material will exceed the ultimate strength of the remaining fin material which will then split through completely. The cut pattern thus achieving the plurality of top base portions, bottom base portions, and fin sets extending between the top and bottom base portions.
By using an chiseling method of forming the cut pattern on the fin stock, the method of the present invention avoids use of shears and lubricants such as in the prior art processes of forming fin arrays.
After the cut pattern is formed on the fin stock, the fin stock is bent by passing the fin stock through a pair of star rollers or other similar device. The fin stock is thus bent into a serpentine pattern so that the top base portions extend in a common top plane, the bottom base portions extend in a common bottom plane, and the fin stock extend between and connect adjacent ones at the top and bottom base portions. At this stage in the manufacturing process, the fin extends at an angle greater than 90xc2x0 with respect to the top and bottom portions to permit the fin stock to roll off of and be removed from the star rollers or other forming device.
After passing through the star rollers, the fin stock is in an uncompressed fin member shape with the fins being angled with respect to the top and bottom base portions. The fin stock is then placed in a compression device where the ends of the fin stock are urged together until the fins extend at an angle of 90xc2x0 with respect to the top and bottom base portions. The manufacturing of the fin stock into the completed one-piece fin member is then complete.
In addition, to avoiding the complexity, cost, and environmental concerns of prior art manufacturing processes, the method of the present invention provides a further advantage in that the impact step of forming the cut pattern in the fin stock results in the fins having serrated or roughened edges. The roughened or serrated edges on the fins have increased surface area on a microscopic level and thus improve heat flux with the surrounding environment.
Other objects, features and advantages of the present invention will become apparent from the following description and drawings in a vehicle